Exercise of specific muscle groups to improve and/or maintain muscular fitness has become fashionable today in the belief that the more effective functioning of the body leads to a more active and fulfilling life style and a more stable psyche. Exercise has therefore become the primary activity of numerous households, corporations, and public and private institutions today.
Typically, exercise can be "dynamic", i.e., including such activities as running or jogging, swimming, bicycling, tennis, basketball, football or skiing, or it can be "static", i.e., including such activities as weight lifting or isometrics.
Where caution is not taken to avoid overindulgence or overexertion before one's body has been properly conditioned, an undesirable, yet almost inescapable consequence, of exercise is injury to bodily tissues, as for example, muscles. Rehabilitative exercise is then necessary to strengthen the injured tissues, and to facilitate maintenance of those tissues newly strengthened.
Of course injury can and does occur on the job and may also involve individuals who let themselves deteriorate as far as muscular fitness is concerned. Their musculature atrophies, weaken and is subsequently injured (through stress/strain) and rehabilitation is necessary. Even fit individuals (athletes etc.) also injure themselves via over exertion during athletic contests, workouts, etc. and also require rehabilitative training.